An electro-hydraulic servo control technology dynamically integrates advantages of a fluid transmission and control technology and an information electronic technology, is applied to important national strategic military industrial fields such as aerospace, sophisticated weapons, steels, power generation or the like, and achieve success rapidly. However, an electro-hydraulic servo valve also has disadvantages of poor anti-pollution capacity, large loss of pressure (7 MPa) in the valve, high manufacturing cost and maintenance cost, and large system energy consumption loss, etc. Because of many disadvantages of the electro-hydraulic servo valve, its quick response performance is unable to be widely used in general industrial equipment. However, a conventional electro-hydraulic switch control is unable to satisfy requirements, in modern industrial production, for a high quality control system. Therefore, it is expected an electro-hydraulic control technology which is low in production and maintenance cost, safe and reliable, and which can satisfy actual demands for an industrial control system in control precision and response characteristic.
Based on the above reasons, an electric-hydraulic proportional technology is put forward. As an representative of the electro-hydraulic proportional technology, an electro-hydraulic proportional valve uses, on a basis of a hydraulic valve used in traditional industry, an reliable and inexpensive electro-mechanical converter (a proportional electromagnet or the like) and a corresponding valve to perform a design, thereby obtaining a proportional control element which is the same as a general industrial valve in requirements for oil quality, small in loss of pressure in the valve, and able to satisfy, in performance, most industrial control requirement.
The electro-hydraulic proportional valve can be combined with an electronic control unit, for conveniently calculating and processing various input and output signals, so as to implement complex control functions. In addition, the electro-hydraulic proportional valve has the advantages of high anti-pollution, low cost and quick response. Therefore, the electro-hydraulic proportional valve is widely used in industrial production, for example, a ceramic floor tile adobe pressing machine, a strip steel constant tension control with a strip mill, a pressure vessel fatigue life testing machine, a hydraulic lift motion and control system, a metal-cutting machine tool workbench motion and control, a rolling mill pressure and control system, a hydraulic punching machine, a pipe bending machine, a plastic injection molding machine and the like.
In a proportional control system, the electro-hydraulic proportional valve not only serves as an electro-hydraulic conversion element, but also serves as a power amplifier element. The electro-hydraulic proportional valve plays an important role in the performance of the proportional control system, serving as a core element of the proportional control system.
The most remarkable feature and the most successful point of the electro-hydraulic proportional valve reside in using the proportional electromagnet as the electro-mechanical converter. Compared with a moving-coil type torque motor and a moving-iron type torque motor, the proportional electromagnet is simple and reliable in structure, good in manufacturability, convenient for use and maintenance, and can output larger force and displacement. In addition to being used for driving a priority valve, the proportional electromagnet also can be used for directly driving a low-powered output stage. For example, a direct-acting proportional valve, which controls a position of the valve core according to a principle that a thrust of the electromagnet is balanced with a spring force, is only applicable to small flow, and the maximum work flow in practical application generally is below 15 L/min (the maximum working pressure is 21 MPa). In addition, in order to achieve a balance of an axial static pressure, the direct-acting proportional directional valve or a flow valve uses a slide valve structure, which may easily be affected by a friction force or oil liquid pollution, thereby causing a clamping stagnation.
A positioning stiffness and a control accuracy of the valve core may be improved to a large extent by using a linear variable differential transformer (LVDT) to conduct a measurement and a closed-loop control of the position of the valve core to constitute an electro-feedback-type direct-acting proportional valve. Meanwhile, a great deal of theoretical study is conducted in terms of model, nonlinearity and system application. Finally, the electro-feedback-type direct-acting proportional valve may be applied, just like a servo valve, to a closed-loop control of a hydraulic system. However, due to magnetic saturation limit, the proportional electromagnet has a limited output force, and thus being unable to fundamentally solve the problem of impact of hydrodynamic force under a high pressure and a large flow, and still having flow saturation in a working condition of a high pressure (large differential pressure) and a large flow.
In order to eliminate the impact of the hydrodynamic force and improve a flow capacity of a hydraulic valve, the most fundamental method is to use a guidance and control (pilot control) technology. As early as 1936, Harry Vickers, an American engineer, invented a guidance and control overflow valve in order to solve a problem that due to impact of hydrodynamic force, a direct-acting overflow valve was unable to implement a pressure control of a high-pressure and large-flow system. The basic thought was as below: a smaller-sized pilot valve is used to control a static pressure, so as to drive a main valve core to move. Because the hydraulic thrust is much larger than a hydrodynamic force generated when oil fluid flows through a valve port, it is enough to eliminate an adverse impact on motion and control of the main valve core. This thought of guidance and control later is widely used in design of other hydraulic valves, so that a high-pressure and large-flow control of a hydraulic system comes true. Subsequently, various electro-hydraulic servo control elements also follow the design thought of pilot control, and the electro-hydraulic proportional valve is no exception, and takes over many structural principles of the servo valve.